The present invention relates to content distribution in packet-switched networks.
Packet-switched networks, such as networks based on the TCP/IP protocol suite, can be utilized to distribute a rich array of digital content to a variety of different client applications. The most popular applications on the Internet today are browsing applications for searching the World Wide Web, e.g. Netscape Navigator or Microsoft Internet Explorer, which utilize the HyperText Transfer Protocol (HTTP) to retrieve documents written in the HyperText Markup Language (HTML) along with embedded content. See, e.g., R. Fielding et al., “Hypertext Transfer Protocol—HTTP/1.1,” IETF Network Working Group, RFC 2616 (1999), which is incorporated by reference herein. Other protocols for delivering data such as streaming media across the Internet include the Real Time Streaming Protocol (RTSP). See, e.g., H. Schulzrinne et al., “Real Time Streaming Protocol (RTSP),” IETF Network Working Group, RFC 2326 (April 1998), which is incorporated by reference herein. Resources on the Internet, such as HTML documents or multimedia content, are identified by Uniform Resource Identifiers (URIs). See, e.g., T. Berners-Lee et al., “Uniform Resource Identifiers (URI): Generic Syntax,” IETF Network Working Group, RFC 2396 (August 1998), which is incorporated by reference herein. URIs can be expressed by a representation of their location dependent network access mechanism, i.e. as a Uniform Resource Locator (URL) (e.g. “http://www.xyz.com/dir/document.html”), or by a persistent name referred to as a Uniform Resource Name (URN).
It is often advantageous when distributing content across a packet-switched network to divide the duty of answering content requests among a plurality of geographically dispersed servers. Companies such as Akamai Technologies, Digital Island, AT&T and Adero provide services—referred to in the art as “content distribution” services—utilizing architectures which dynamically redirect content requests to a cache advantageously situated closer to the client issuing the request. Such network architectures are referred to herein generically as “content distribution networks” or “CDNs” for short. In its simplest form, content distribution networks consist of origin servers and edge servers. Clients connect to edge servers to request content. Requested content may already be in the edge server that the client connect to (for example if all of the edges are pre-populated with all of the content), or the edge server in question might fetch the content from the origin server on-demand if it does not already have the requested content. These two extremes, namely complete pre-population of the edges and on-demand loading of the edges, is clearly suboptimal in terms of storage and latency respectively. This is particularly true in the case of high quality streaming content where storing all of the content in all of the edge servers will not be feasible. At the same time if say a particular large piece of digital content, e.g. a movie, is already at an edge server, a new client requesting the same movie might potentially be best served from the same edge server.